Ammonia is highly toxic to aquatic organisms, but whether ammonia excretion or ammonia metabolism to less toxic compounds is the major strategy for detoxification in marine fish against chronic ammonia exposure is unclear to date. In this study, we investigated the metabolism and excretion of ammonia in marine medaka Oryzias melastigma during chronic ammonia exposure. The fish were exposed to 0, 0.1, 0.3, 0.6, and 1.1 mmol l(-1) NH4Cl spiked seawater for 8 weeks. Exposure of 0.3-1.1 mmol l(-1) NH4Cl had deleterious effects on the fish, including significant reductions in growth, feed intake, and total protein content. However, the fish could take strategies to detoxify ammonia. The tissue ammonia (T-Amm) in the 0.3-1.1 mmol l(-1) NH4Cl treatments was significantly higher than those in the 0 and 0.1 mmol l(-1) NH4Cl treatments after 2 weeks of exposure, but it recovered with prolonged exposure time, ultimately reaching the control level after 8 weeks. The amino acid catabolic rate decreased to reduce the gross ammonia production with the increasing ambient ammonia concentration. The concentrations of most metabolites remained constant in the 0-0.6 mmol l(-1) NH4Cl treatments, whereas 5 amino acids and 3 energy metabolism-related metabolites decreased in the 1.1 mmol l(-1) NH4Cl treatment. J(Amm) steadily increased in ambient ammonia from 0 to 0.6 mmol l(-1) and slightly decreased when the ambient ammonia concentration increased to 1.1 mmol l(-1). Overall, marine medaka cope with sublethal ammonia environment by regulating the tissue T-Amm via reducing the ammonia production and increasing ammonia excretion. (C) 2017 Elsevier Ltd. All rights reserved.